Strong absorption lines in the mid-IR region from the vibration of chemical bonds can be used to identify important molecules. Mid-IR wavelength-tunable sources like distributed-feedback (DFB) quantum-cascade lasers (QCLs) or wavelength tunable external-cavity (EC) QCLs may be used to scan the wavelength around an absorption line. DFB QCLs are often used to detect a selected narrow absorption peak, such as the major absorption peak of a particular small molecule, for example, the absorption peak of CO2 near a wavenumber of 2350 cm−1 (4.2-4.3 μm) on the horizontal axis, shown on a y axis of line intensity (cm−1)/(molecule cm−2) in the graph of FIG. 1. Existing EC-QCLs generally have a significantly larger wavelength tuning range of around 100 and even up to 500 cm−1 and may be used to detect the broader characteristic absorption profile of a large molecule. One previously proposed example is the detection of the absorption profile from 950-1200 cm−1 of glucose, as shown in the graph of FIG. 2 (see, e.g., M. Brandstetter, A. Genner, K. Anic and B. Lendl, “Tunable external cavity quantum cascade laser for the simultaneous determination of glucose and lactate in aqueous phase,” The Royal Society of Chemistry, 2010), with traces showing absorbance as a function of wavenumber for 300, 200 and 100 mg/dl glucose concentrations, downward from the top trace, respectively. Another example of interest is the detection of the mid-IR absorption profiles characteristic of common explosive materials. Using an external cavity QCL, Fuchs et al. demonstrated the capability to detect quantities of explosives like PETN, TNT, RDX, and SEMTEX with sensitivity 20 μg/cm2 at distances up to 25 m. (See F. Fuchs, S. Huggera, M. Kinzera, Q. K. Yanga, W. Bronnera, R. Aidama, K. Degreifb, S. Rademacherb, F. Schnürerc, and W. Schweikertc, “Standoff detection of explosives with broad band tunable external cavity quantum cascade lasers,” Proc. of SPIE vol. 8268, 82681N-1-9, 2012.) The graph of FIG. 3 shows absorbance as a function of wavelength in micrometers for PETN (102), RDX (104), TATP (106), and TNT (108), together with one example of a wavelength range R of interest for used in detecting these and other substances.
EC QCLs typically offer a wide tuning range, but are larger and more expensive to fabricate and maintain than DFB QCLs and are relatively slow to tune to a given frequency with typical tuning time in the millisecond range. DFB QCLs typically are smaller, less expensive to manufacture and maintain, and typically allow for the emission of stable, narrow wavelengths that are very quickly tunable, in typically very narrow ranges, with tuning time in the microsecond range. Realization of readily manufacturable widely tunable QCLs such as DBR lasers with superstructure grating has been lacking, however, preventing their economical application in sensing larger molecules such as those with absorption profiles like the ones depicted in FIGS. 2 and 3 above. It would be advantageous in various fields, if the low cost, relative efficiency, quick-tuning and other beneficial properties of superstructure grating DBR QCL lasers could be readily and practically utilized across a broad range of wavelengths.